Love, SquaredWith a streamlined adaptation complete, the filmmakers could turn their attention to finding a young actress with enough depth and skill to create Melanie-Wanda, a character with one body and two voices. "When we discussed the idea of the dual consciousness with Andrew, we were pleased that he completely agreed with us," says Wechsler. "He thought it was strictly a matter of performance."

Around that time, Wechsler attended a screening of Hanna, with the young Irish actress Saoirse Ronan in the title role. "About 20 minutes into the movie, the hair on the back of my neck stood up, because Saoirse Ronan was clearly the one," he says. "There weren't many people I thought could pull off playing two characters in one body, but she could."

He contacted Meyer and the other filmmakers, urging them to see the movie as soon as possible. "I'd always pictured the character being between 25 and 30. But watching Saoirse in Hanna immediately changed my mind. She can do anything and we needed an actress who could play two really different characters. Melanie is all action and so tough, while Wanda embodies the peaceful Soul radiating calm and kindness."

Niccol says they never seriously considered another actress for the role. "We didn't have a Plan B. Once I'd seen Saoirse in Hanna, I knew I wanted her to play this part. There's something inherently truthful about her. I don't know anyone else who could play Wanda and Melanie with as much empathy as Saoirse."

"It's a bravura performance as anyone who sees it will discover," says Steve Schwartz. "Not only does she pull it off, she makes it look easy."

Ronan, who was 17 when shooting for The Host commenced, began her film career at the age of 9. In 2007, when she was just 13, she earned Oscar, Golden Globe and BAFTA nominations for her role as Briony Tallis in Atonement, opposite Keira Knightley and James McAvoy. Already familiar with Meyer's work from the Twilight series, she quickly read the script that Wechsler sent and talked it over with her dad, actor Paul Ronan.

"We were both really excited by the idea of playing two characters in one film," she recalls. "It's an actor's dream. I was also very intrigued by the story, as well as the idea of working with Andrew Niccol. Every project he's involved in seems to have a mind-blowing concept behind it. And of course I was excited about working with Stephenie. She was on set almost all of the time and involved with everything."

"It's a different kind of story for Stephenie," the actress observes. "It's not a love triangle exactly -- it's more a love square because there are four people involved, two in the same body. They're both in love with different people, which makes it complicated."

Finding a way to differentiate between two characters inhabiting the same body began with creating a distinctive voice for each. "Saoirse is Irish and she had to play two American accents, one slightly southern from Louisiana and a more generic American one," says Niccol. "In addition, Wanda is new to the language and the planet. In the beginning, she speaks quite formally, but then learns irony and sarcasm and even the ability to lie from her human host."

Ronan developed two individual ways of moving. "The walk for each character became important to me. Wanda's very delicate. She almost floats, whereas Melanie is tough and feisty. I tried to bring that out in the way they hold themselves and even small things like hand gestures."

Meyer predicts audiences will be astonished by Ronan's sensitive balancing act. "I hope as many people as possible see her in this role," the author says. "It's exciting to imagine where her career will go over the next few years. I can't wait to see what her next movie is."

With the film's most critical role filled, the filmmakers turned their focus to finding the actors who would play the men in her life. The search for Melanie's and Wanda's love interests was extensive and Ronan was involved in the casting process for her two leading men from the get-go. "It was essential that we all work well together," she says. "It's the first time I've been a part of an on-screen romance and I was lucky enough to do it with two people I love being with. Andrew and I were in London when Max Irons, who plays Jared, came in. I was delighted because I already knew him.

"The same thing happened when we met with Jake Abel for the role of Ian," Ronan continues. "We worked together on a film called The Lovely Bones. It was the first time I had played romantic scenes in a movie and already knowing Jake and Max made it so much more comfortable. Each time, as soon as they left the room, Andrew and I looked at each other and said, 'It has to be him.'"

Wechsler adds: "We tested as many guys as we needed to until we found the right chemistry. It was obvious right away that Max Irons and Jake Abel were the ones. Max is on the rise professionally. Like so many actors in this piece, we got him just as he is turning the corner from boy to man."

Irons, who recently appeared opposite Amanda Seyfried in Little Red Riding Hood, has a lengthy show-business pedigree that includes parents Jeremy Irons and Sinead Cusack, as well as grandfather Cyril Cusack. At his initial meeting, the actor was ready to give up almost before he started. "Auditioning for Andrew and Saoirse was terrifying," says Irons. "While I was waiting, there were three guys sitting there, all looking like Greek gods. I thought, well, what's the point, and forgot my lines about eight times. But the chemistry certainly was there with Saoirse, which felt great.

"She is always spot on and perfectly informed," he continues. "It still amazes me that she was 17 when we shot the film. I worked most closely with her and with William Hurt, two masters of their craft, and both were so humble, patient and generous."

Irons' character is facing several almost insurmountable obstacles. "First, humanity as we know it has been wiped out," he explains. "He has that enormous truth to deal with, as well as knowing that the love of his life, Melanie, has become a Soul. To him, she is dead. When Wanda suddenly appears, it is like a ghost returns. Despite the fact that she's a Soul, it still looks like the girl he loves. Logic goes out of the window and he is operating on gut and instinct and confusion."

Like many of the people involved with The Host, Irons is a dyed-in-wool science-fiction fan, but the philosophical questions the film raises were more interesting than the fantasy elements to him. "Would Earth be better without the Souls? The more we learn about them, the more it becomes apparent that, despite the fact that they are essentially parasites, their intentions are much more than that. Are we actually our own worst enemies? If there's anyone who can write this convincingly, it's Stephenie. She's in touch with something that a lot of other people can't quite get right."

Throughout the casting process, Meyer was concerned that Ronan's unforced charisma threatened to eclipse her co-stars. "But the chemistry between Max and Saoirse was just unbelievable," she says. "He's able to do so much without saying a word, so you will actually pay attention to him while Saoirse is in the same shot."

Jake Abel, who has a central role in the Percy Jackson franchise, shares that quality, says the author. "I've always loved Jake as an actor. He steals his scenes. You watch him instead of the people you're supposed to be watching. That kind of presence makes him a really strong leading man."

The actor says that his character, Ian, distrusts Wanda immediately and would be happy to dispatch the Soul quickly and quietly. Instead he finds himself falling in love with his mortal enemy. "Ian has no fealty to Wanda," says Abel. "She has to go. She's a risk to our security and our livelihood. But the more he gets to know her, the more he sees that she's more human than humans are. Her generosity, her love and her kindness make Ian fall for her. The idea of an interspecies love affair was fun to explore."

"Jake Abel is one of the few young actors who could convince me that he had fallen in love with an extraterrestrial being," says Niccol. "You can talk about falling in love with someone's spirit, but this is a literal example of that."

Abel makes the transition from dangerous antagonist to selfless romantic with grace, says Meyer. "In the book, he was a sensitive, intellectual guy, as opposed to a real sturdy man like Jake, but Andrew's idea was to flip the character on its head and have the guy's guy be the one who falls for an alien creature inside a human. Nobody would expect that."

Another central character of The Host is the Seeker, a Soul who tracks humans and inserts other Souls into their bodies. Played by Diane Kruger, who has appeared in Quentin Tarantino's Inglourious Basterds and the National Treasure series, this particular Seeker is a bit of an anomaly, lacking the serenity and detachment of her peers. Tormented and driven, she becomes obsessed with unlocking Melanie's memories and discovering the whereabouts of her surviving loved ones.

"Diane Kruger was born to play the part," says producer Steve Schwartz about the character. "Her fierce determination is mesmerizing."

"The Seeker had to stand up to Saoirse," says Meyer. "If you've seen Hanna, you understand how challenging that is. Diane and Saoirse are great together. Diane can be so icy and at the same time maintain a warm veneer. She can project a menacing presence when she wants. I thought it was nice to see these two women as the Yin and the Yang, the scary and the strong."

The German-born Kruger is white-blonde perfection as Melanie-Wanda's dangerous opponent. "I don't think Diane has ever played a villain before," says Niccol. "She has a complexity, which the role needed. There had to be more going on than just playing a bad guy. The Seeker is truly scary because she's killing with kindness. When she catches a human being, she views it almost as an intervention."

Kruger says her taste for science fiction made the offer to participate in The Host irresistible. "I was a huge fan of Gattaca, so working with Andrew was a big draw. I love his aesthetic. The movie looks really cool, sleek and modern. And his love for detail is impressive. He's the kind of director who will move a glass an inch further to the left because he just knows it will look better there."

Her character's conflicted motives made the Seeker especially intriguing to play, adds the actress. "Her journey in the movie is really interesting. These aliens are not necessarily the bad guys. Yes, they have invaded the planet and taken over human bodies, but in a way they are perfecting our world. And yet the human mind and the human spirit are so strong that even they have difficulty overcoming that."

The filmmakers were thrilled to land Oscar winner and four-time nominee William Hurt for the role of eccentric rebel leader Uncle Jeb. "William Hurt is on the Mount Rushmore of American actors," says Niccol. "He's a national treasure and he grounds the movie. Like Saoirse, he is unable to do anything that is not honest."

Wechsler says watching Hurt play a role of substance was an exhilarating experience. "William is one of the greatest male actors of all times. And he's been too little seen in the last 10 years. He has a lot in him that will surprise the audience, but what surprised us was how he responded to the material.

"He talked to us about man's relationship to other species, even species we don't have on this planet, and about our relationship to the planet and the universe," the producer continues. "William thinks about those things, so he flipped for the script and was very passionate about being involved."

Hurt's initial reaction can be summed up in two words: "wonderful script."

"There are many, many interesting elements to the screenplay," he says. "I leapt at the chance to participate because I loved Andrew's writing. It's also a privilege to be working with a group of great young actors who are really enthusiastic, open, intelligent, skilled and disciplined."

For their part, the younger actors were inspired by Hurt's enthusiasm and talent. "I loved working with William," says Ronan. "He is such a gifted actor. He was always asking questions, because he truly wants to understand. That helped all of us. William would ask a question about something I thought I had all figured out and the scene suddenly became better."

Also part of the human resistance is Jeb's sister and Melanie's aunt, Maggie, played by Frances Fisher, whose impressive resume includes playing Ruth Dewitt Bukater, Rose's mother, in James Cameron's epic, Titanic. "Frances brings something new to every take," says Niccol. "She has an unexpected quality to her. In each scene, she came in with an idea that took me by surprise. I try to always be open to happy accidents and Frances is the kind of actor who provides them."

"Having Frances and William tells audiences that this not a young-adult popcorn movie," adds Wechsler. "It is a movie with depth and breadth that will appeal to everybody."

Maggie is very suspicious of this creature that looks like her niece, but has the consciousness of a Soul. "Frances is a very sweet woman," says Ronan. "She's so maternal. But Maggie is not the nicest person in the world. It was hard to believe that someone so lovely could pull it off, but she did."

One of Melanie's major motivations for getting back to her people is her younger brother, Jamie, whom she has protected since their father took his own life rather than be taken by the Souls. The producers watched dozens of video auditions before discovering Chandler Canterbury, who appeared in The Curious Case of Benjamin Button as Benjamin at the age of eight. Meyer admits it was rough saying no to so many talented young kids, but Canterbury was the hands-down choice. "His audition was amazing," she says. "The emotion felt so real that your heart just broke for him."

Niccol agrees: "He is such an authentic young actor. He could cry on cue. Frankly, he was a revelation."

Surveying a cast divided between movie veterans and rising stars, Wechsler is 100 percent pleased with the casting choices. "What I know for sure about this movie is that the acting is superb," he says. "The cast cared so much about their performances and that enabled them to fully inhabit all of the dramatic moments of the film."